This invention relates to an injection molding method using a surge pressure to be given to a molding material injected into a mold cavity, and more particularly to an injection molding method in which a surge of high pressure is momentarily applied to the plasticized resin material injected into the mold cavity immediately after the completion of a resin filling process so as to suppress molding shrinkage and orientation of the resin material which occur in the course of solidification of the material, to thereby fulfill super-precision molding.
Recently mass-storage memory media such as optical discs and audio compact discs are being increasingly put into practical use. It is undeniable that the development of precision plastic molding techniques for molding such optical discs makes a contribution to the practical application of high-density recording optical discs of high performance. For realization of high-density recording on the optical disc, it is essential to not only finish accurately the optical disc on the submicron order, but also achieve uniform optical characteristics of the optical disc.
Various methods have been developed for precisely molding the high-density recording optical discs. For instance, the inventor of this invention has proposed a basic system for precise injection molding by use of internal die pressure of a molding resin material injected in a mold cavity (Japanese Patent Publication (B2) SHO 58(1983)-52486). In this conventional system, the internal die pressure is continuously monitored and controlled during injection molding in accordance with reference pressure predetermined in a waveform pattern, to thereby produce remarkably precise molded articles.
The inventor further proposed a high-speed injection molding method on the basis of the aforesaid injection molding system in U.S. Pat. No. 4,797,236, in which a molding material is injected in an injection mold under a high vacuum at an ultrahigh speed.
Now, in order to improve the molding precision, molding shrinkage which inevitably occurs in plasticized material injected in the mold cavity in the course of solidification of the material on cooling should be compensated thoroughly. In a compression molding method, "dwelling" is often effected for compensation of the molding shrinkage in the molding material.
One of the compression molding methods has been proposed in U.S. Pat. Application Ser. No. 06/624,968, now U.S. Pat. No. 4,863,651, in which compression pressure to be applied to an injection mold is regulated with mold-clamping force while controlling internal die pressure of the plasticized resin material injected in the mold cavity in conformity with predetermined reference internal die pressure.
These conventional molding methods, however, could not sufficiently respond to the strict requirements imposed on the precision molding. That is, a technical system for thoroughly compensating the molding shrinkage caused in the plasticized resin material on cooling as mentioned above has not been established so far.
To be more specific, the solidified part of the molding material in the mold cavity spreads from the contact surface portion in touch with the inner surface defining the mold cavity toward the inside of the molding material. The molding material molten by heating progresses in solidification of the material even during a short period of resin filling and compression processes. That is to say, since solidification and shrinkage of the molding material within the mold cavity have already started when the compression process has begun to apply compression pressure to the molding material in the mold cavity by means of a mold clamping device, the molding material which is being solidified on cooling is little influenced by the compression pressure after the filling process is finished. Thus, it is preferable to compress the molding material in the mold cavity before the molding material begins to solidify, whereas the application of the compression pressure before solidification of the material will give rise to back-flow of the molding material to an injection part from which the molding material is introduced into the mold cavity.
In addition, the conventional molding methods cannot decrease the directional property resulting in "orientation" remained inside a finished molding, which is caused by subjecting the composition of the molding material to thermodynamic movement during molding. The orientation in the finished molding brings about unevenness in thickness and birefringence of the finished molding. For instance, audio compact discs of 12 cm in diameter and 1.2 mm in thickness actually produced by a conventional injection molding method have by and large an error of about 20 .mu.m to 30 .mu.m in thickness and are uneven in birefringence.